US2386049A - Apparatus for converting sound to frequency modulated currents - Google Patents

Description

W. HAUSZ Oct. 2, 1945.

APPARATUS FOR CONVERTING SOUND TO FREQUENCY MODULATED CURRENTS Filed March 9, 1943 Inventor: Walter Hausz, y 36 6 His Attovne g Patented Oct. 2, 1945 APPARATUS FOR CONVERTING SOUND TO FREQUENCY MODULATED CURRENTS Walter Hausz, Schenectady, N. Y., asslgnor to General Electric Com New York pany, a corporation of Application March 9, 1943, Serial No. 478,565

11 Claims.

My invention relates to sound reproducing apparatus, and it has for one of its objects to provide an improved honograph pick-up apparatus for converting recorded sound waves to frequency modulated electric waves.

My invention relates more particularly to phonograph pick-up apparatus of the type in which one electrode of a variable capacitor is caused to vary in response to the amplitude of motion of a stylus which cooperates with or engages the sound record to be reproduced. It is desirable that such apparatus be of light weight in order to reduce wear on recordings being used and to provide a constant response in the output of the apparatus for a constant amplitude of needle motion over the frequency range of the recorded sound waves.

A further object of my invention is to provide an improved pick-up apparatus of the above type in which the driving force of the record on the pick-up apparatus is maintained at a minimum value over the entire useful frequency range of the recorded sound waves.

Another object of my invention is to provide an improved phonograph pick-up apparatus for converting recorded sound waves to frequency modulated electric waves in which response to constant and low frequency forces on the apparatus and due to eccentricities in the recording is substantially eliminated.

A still further object is to provide an improved sound pick-up apparatus in which excessive resonance in the useful frequency range is substantially avoided.

One of the features of my invention is the use of a sound wave pick-up apparatus as means for frequency modulating a pair of electron discharge devices, connected in push-pull and oscillating at different frequencies, and utilizing the detected difference frequency of their output as a source of frequency modulated carrier waves. Another object of my invention, therefore, is to provide an improved frequency modulation system in which distortion of the output wave caused by harmonics of the modulating signal is reduced to a very low level.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fi 1 is a pick-up apparatus constructed (Cl. I'm-100.4)

according to my invention; Fig. 2 is a sectional view of the apparatus of Fig. 1 along the line 2-2; Fig. 3 shows an element of the pick-up device of Fig. l; and Fig. 4 is a circuit diagram of a portion of a frequency modulation system utilizing the pick-up device of Fig. 1.

Referring to the drawing, I have shown in Fig. 1 a light weight phonograph pick-up of the variable capacity type and comprising a variable plate or electrode I0, a pair of fixed or stator electrodes H and I2, and a needle 13. The electrode I0 comprises a sheet of elastic material having a pair of conductive surfaces and formed with a pair of end members or legs l5 and 16, a central member or leg I l, and a cross member I8 connecting the legs l5 and 16 with the central member ll. The plate l0 may be formed from a sheet of elastic metal, such as beryllium copper for example, or may comprise a thin layer of plastic material of the synthetic resin type having surfaces made conductive by spraying or plating with a conductivematerial, such as graphite. The legs l5 and I6 may be formed by cutting a pair of slots, one of which is indicated at IS in Fig. 1 and both of which are indicated at l9 and 20 in Fig. 3, in the sheet of material and drilling a pair of holes 2| and 22, as indicated in Fig. 3, at the base of these slots to prevent local concentration of stresses in the material as the central member l! is flexed in a manner pointed out later. The leg members l5 and I6 likewise extend vertically from the cross member I8 a greater distance than does the central member H. In this manner the legs l5 and I6 provide means for rigidly attaching the variable plate to the usual phonograph pick-up arm 24. The plate It), likewise, is formed with an edge 25 bent at right angles to the cross member I8 to give rigidity to the plate, as well as to provide means for attaching the pick-up needle l3 to the plate 30. Preferably, the needle i3 is of very light construction and may comprise, for example, a sapphire needle integrally attached to the edge or stiffening member 25.

The stator plates H and I2 are formed of a substantially rigid material having conductive surfaces, such as a metal plate or a plastic plate, thicker than the member l0 and provided with a conducting surface in the same manner as the plate Ill. The plates H and l2 are of substantially the same size as the central member I! and are supported in spaced parallel relation with member I! by means of a pair of insulating members 30 and 3|. The members 3}] and a minimum of oscillations, and may comprise a synthetic resin, such as, for example, butyl rubher. The insulators 30 and 3| may be attached to the plates ID, i! and I2 by any suitable method, such as by vulcanization. Leads 32 and 33 are attached to plates II and I2 to provide means for connecting these plates to the electrical circults of thepick-up.

In the construction thus far outlined, the plates Ill and II and the plates in and I2 form a plurality of capacitors, the capacitances of the individual capacitors being varied in opposite sense by movement of the variable plate I as the needle I3 moves along the usual sound track 35 of a phonograph record. Movement of the needle I! along the track 35 causes the central portion I! of plate ill to vibrate at audio frequencies. Preferably, the mass of the plates H and I2 and the insulators 30 and 3! is such that the natural frequency of vibration of these plates in conjunction with the plate occurs at a frequency below the range of frequencies of the usual audio impulses on a phonograph record. Thus, at frequencies in the range of the audio .impulses, the top of the mass comprising the stators H and I2, the insulators 3d and 3| and the upper portion of member l'l, remains substantially fixed in space and only the lower portion of plate It moves in response to the audio frequency impulses imparted to needle l3 by sound track 35. Impulses of sub-audio frequencies at or near the natural frequency do not cause excessive motion of plates H and I2 by virtue of the damping properties of insulators 30 and 3|.

While the plates II and I2 do not move in response to frequencies in the audio range, application of a constant force or a force of low frequency to needle I3, because of the presence of insulators 30 and 3 l is effective to move the plates III, II and I! as a unit. In this manner application of a sub-audio frequency force, which may be caused by off-center records or other eccentricities in a recording, does not change the capacitances of the component capacitors of the pick-up apparatus. At frequencies considerably above the natural frequency of plates II), II and l2, a constant amplitude of needle motion produces a constant percentage change in capacity. At high frequencies in the audio range, moreover, the response is of substantially the same order as for corresponding forces at lower frequencies in the audio range because of the construction of the pick-up device. Thus, it is apparent that the only mass which is moved by the vibrating needle I3 is the mass of the needle plus a small portion of the mass of the vibrating plate or reed l0. In this way, over the entire useful frequency range, the driving force required to obtain a desired capacitance change is kept to a value. The construction outlined, moreover, avoids all violent resonances of the vibrating plate even for frequencies outside of the useful frequency range, since the insulators 30 and 3| are chosen to have suflicient damping because of their high mechanical hysteresis properties.

In the circuit of Fig. 4, there is shown a portion of a frequency modulated high frequency apcircuits caused by harmonics of the audio impulses. In the circuit, electron discharge devices 40 and 4| are two oscillators providing high frequency oscillations determined by the tuned oscillatory circuits connected between the anodes and control electrodes of these devices. For reasons which are pointed out later, these oscillatory circuits, which comprise the inductances 42 and 43 and oapacitances 65 and 66, are tuned to different frequencies. One terminal of the inductances 42 and 43 is grounded and connected through capacitor 44 and load resistances 45 and 46, respectively, to the anodes of devices 40 and M. The opposite terminals of these inductances are connected through grid resistors 41 and grid by-pass capacitors 48 to the control electrodes of these devices and intermediate points 49 on inductances 42 and 43 are connected to the cathodes of the devices. Operating potential for the anodes of devices 40 and 4! may be supplied from any suitable source, such as the battery 5|).

The capacitances 65 and 66 are formed by the plates IO, N and I! of the pick-up devices. In the absence of audio impulses to vibrate the plate ill, the devices 40 and 4| individually function as the usual type of single tube oscillator in which a portion of the energy in the anode circuits is fed back to the input circuits to maintain oscillations in the output circuits of the tubes. Since the capacitors 65 and 66 form part of the tuned circuits of oscillators 4|) and 4|, when the reed or common plate I0 is moved to one side, for example to the right-hand side, by a sound impulse imparted to needle l3, the frequency of oscillator 40 increases and that of oscillator 4| decreases. In this manner the oscillations in the output cirsuits of oscillators 4| and 4B are frequency modulated by audio impulses supplied to the needle l3.

In order to obtain a. minimum of harmonic distortion in the output of the frequency modulated oscillators 40 and 4 I, the anodes of devices 40 and 4| are coupled by means of capacitors 52 and 53 to a resistor 54, the mid-point of which is connected to ground through a resistor 55. A diode 55 is likewise electrically connected between the mid-point of resistor 54 and ground through a load resistor 51. This diode 56 functions in a well known manner as a detecting circuit to develop across resistance 51 voltages of the frequency of oscillations of oscillators 40 and 4|, as well as voltages having frequencies equal to the sum and difference of frequencies of the oscillations of oscillators 40 and 4|. In order to remove from the output terminals 60 and 6| of the frequency modulation system unidirectional voltage components, voltages of the frequencies of oscillators 40 and 4|, and voltages having frequencies equal to the sum of the frequencies of these oscillations, a filter circuit comprising inductance 62 and capacitances 63 and 64 is connected across resistance 51 between the cathode of diode 55 and ground. The resulting voltages supplied to the output terminals 60 and BI contain the difference frequency between the two oscillators. This frequency is lower than the frequency of either of the oscillators and these oscillations are frequency modulated in accordance with the difference in capacity which occurs between the two pick- up capacitors 65 and 66 as the reed I0 is vibrated at an audio rate. One of the advantages of this particular system is that, since a larger amount of modulation is 'ob'-" tained for a given capacity change, the use 'of vibrations harmonics of the audio signal present on sound track 35 in the output of oscillators 4| and 40 by the push-pull connection of capacitors 65 and 66 in the tuned oscillatory circuits.

It is, of course, apparent that since the two oscillators 40 and 4| differ in frequency, the desired output frequency for perfect compensation and cancellation of even harmonics in the output of the oscillators is achieved only by properly constructing the capacitors 65 and 66 or by choosing the additional tuning capacitors 61 and 68 so that they have capacitances proportioned to the desired frequencies. In general, it may be necessary to use the additional tuning capacitors 61 and 6B in order to obtain oscillations of desired frequencies. In specific cases, one or both of these capacitances may be reduced to zero value.

While I have shown a pick-up device in which two stators H and B2 are used in conjunction with a vibrating element ill, it is apparent that the principles pointed out in connection with the construction and operation of the pick-up of Fig. 1 may be applied equally as well to a case in which a single stator element is used in conjunction with the vibrating reed. In a frequency modulation system using a pick-up device having a single stator plate, of course, a single oscillator is used rather than a plurality of oscillators Gil and ll shown in Fig. 4. In such a system, harmonic distortion in the output may be reduced by increasing the separation between the stator element and the reed element to reduce the per cent capacity change for vibrations of a given amplitude.

While I have shown particular embodiments of my invention, it will, of course, be understood that I do not wish to be limited thereto since various modifications may be made, and l contemplate by the appended claims to cover any such modifications as fall within the true spiritand scope of my invention.

What i" claim as new and desire to secure by Letters Patent of the United States is:

i. In combination, a member vibrating at audio frequencies, a pair of oscillatory circuits tuned to different frequencies, means for frequency modulating oscillations of said circuits by the vibrations of said member comprising a pair of capacitors having a common electrode, each of said capacitors being connected in a respective one 01 said circuits, means to vibrate said common electrode inaccordance with the vibrations of said member, and means coupled to said circuits for deriving therefrom voltages of a frequency equal to the difference in frequency of said oscillations.

2. Apparatus for converting audio frequency into frequency modulated waves comprising a pair of electron discharge oscillators having input and output circuits, said output circuits being connected in push-pull, a respective one of a pair of tuned oscillary circuits coupled to the input circuit of each of said oscillators, each of said oscillatory circuits being tuned to a different frequency and including a capacitance, means responsive to said vibrations for varying said capacitances in opposite sense for frequency modulating said oscillators, and means coupled to said output circuits for detecting the difference frequency of said modulated oscillations.

3. The combination, in a capacity pick-up device for phonographs, of a pair of electrodes, one of said electrodes being supported from the other of said electrodes by insulating means, a stylus attached to the other of said electrodes adapted to engage a record to be reproduced and to vibrate said other electrode in accord with said record to be reproduced, both of said electrodes and said insulating means being moved in unison by motions of said stylus at frequencies below the frequencies involved in said record to be reproduced, and said electrodes having a mass and said insulating means having mechanical hysteretic properties such that said other electrode vibrates relative to said one electrode in response to motions of said stylus at frequencies involved in said record to be reproduced.

4. In combination, a variable capacitor comprising a thin flexible plate having a pair of conductive surfaces, said plate comprising a pair of end members and a central member connected by a cross member, a pair of substantially rigid plates in parallel spaced relation with said central member, means for supporting said rigid plates from said central member, said means including insulating members located between a corresponding edge of said central member and said plates, means attached to said cross member for moving said central member with respect to said rigid plates at audio frequencies, and means whereby said capacitor may be supported from said end members.

5. A pick-up device for the reproduction of recorded impulses comprising a fiat plate of thin flexible material having a conductive surface, said plate comprising a pair of legs connected together at one end, a substantially rigid plate having a conductive surface in spaced parallel relation with one of said legs and forming therewith a capacitance element, an insulating member joining said rigid plate and the end of said one leg opposite said one end, said rigid plate being supported solely by said one leg through said insulating member, means attached to said one end for actuating said one leg to vary the capacitance of said element in response to said impulses, and supporting means attached to the other of said legs at the end opposite said one end.

6. The combination, in a phonograph pick-up apparatus, of a pair of capacitors having outer electrodes and a common electrode positioned between said outer electrodes, said electrodes being fixedly attached together along one edge thereof and insulated from each other, and all of said electrodes being movable together about said one edge in response to movement of the opposite edge of said common electrode at frequencies lower than the frequencies to be reproduced, a stylus attached to said opposite edge adapted to engage the record to be reproduced and to vibrate said opposite edge in accord with said record thereby to vary differentially the capacitances of said capacitors at the frequencies to be reproduced, and means to produce electric waves varying with the difference in said capacitances whereby said waves vary in accord with the record to be reproduced and are unaffected by low frequency forces affecting said stylus.

7. The combination, in a capacity pick-up device for phonographs, of three parallel electrodes having capacity therebetween, a stylus attached to the, intermediate one of said three electrodes to vibrate said electrode between th other two of said three electrodes in accord with the record to be reproduced, and a mechanical coupling between said electrodes to vibrate both of said two electrodes in unison with said interaccord with the record to be reproduced thereby to vary oppositely the capacities between said intermediate plate and the respective other of said plates, and a mechanical coupling between said plates to vibrate all of said plates in unison and in phase in response to vibration of said intermediate plate at frequencies lower than frequencies to be reproduced thereby to reduce capacity variations in said capacities at said lower frequencies.

9. The combination in a pick-up device, of a pair of plates, one of said plates being supported at one edge thereof and carrying a stylus at the other edge, means insulatingly supporting said other plate from said first plate to vibrate in unison therewith at a certain frequency, said means including means whereby said stylus vibrates said first plate independent of said other plate at higher frequencies.

10. The combination, in a pick-up device, of a pair of conductive plates, one of said plates being supported at one edge thereof and carrying a stylus at its other edge to engage a record to be reproduced, means insuiatingly supporting said other plate from said one edge in spaced parallel relation with said first plate, whereby said plates vibrate in unison, at frequencies below a certain frequency, said means including means whereby said stylus vibrates said first plate independent of said other plate at frequencies greater than saidfirst frequency.

11. The combination, in a capacity pick-up device for phonographs, of a pair of electrodes, one of said electrodes carrying a stylus adapted to engage a record to be reproduced and mounted for vibration relative to said other electrode in accord with said record to be reproduced, thereby to vary the capacity between said electrodes in accord with said record, and means to prevent relative movement between said electrodes in response to slow motion of said stylus at frequencies below the frequencies involved in the record to be reproduced, said means comprising'means to support said other electrode from a portion of said one electrode remote from said stylus whereby the portions of both electrodes near said stylus vibrate freely.

WALTER HAUSZ.

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